1. Field of the Invention
The present invention relates to a method of quantitative analysis of hexavalent chromium in a coating for a metal substrate such as a chromate coating used in household electronic appliances and. automobiles. The present invention also relates to a method for controlling a hazardous element in an encapsulating resin of a resin encapsulation semiconductor device with a fluorescent X-ray analyzer.
2. Description of the Related Art
In Europe, it is required to restrict the use of lead, mercury, cadmium, PBB, PBDE, and hexavalent chromium in principle in accordance with the Restriction of Hazardous Substances Directive (ROHS) which takes effect on Jul. 1, 2006. In order to comply with the directive, it is desired to develop analytical methods capable of conveniently assaying these substances.
It is known that X-ray photoelectron spectroscopy can be used as an analytical method for directly assaying hexavalent chromium content in a chromate coating (see Jpn. Pat. Appln. KOKAI No. H05-164710). Since this method assays only the surface of the coating, however, there are problems that the analytical value does not represent the value for the entire sample and that it is hard to separate peaks of trivalent chromium and hexavalent chromium.
ISO 3613 defines an analytical method for determining hexavalent chromium eluted from a chromate coating using boiling water. This method assays only hexavalent chromium eluted by the boiling water, and cannot determine total hexavalent chromium content present in the chromate coating.
An analytical method using sodium hydroxide solution as an extracting solution is known in “Method 3060” defined by the United States Environmental Protection Agency (EPA). When a chromate coating on a substrate of an amphoteric metal such as aluminum is assayed using the extracting solution, the aluminum substrate is eluted prior to the chromate coating accompanied by hydrogen gas generation and reduction of hexavalent chromium to trivalent chromium, making it difficult to perform highly accurate quantitative analysis of hexavalent chromium.
Further, as an analytical method of efficiently extracting hexavalent chromium in a chromate coating on a metal substrate in a short time, a method is known in which the chromate coating is cracked and then the coating is immersed in an extracting solution to extract hexavalent chromium so as to be analyzed (see Jpn. Pat. Appln. KOKAI No. 2004-325321). Since this method requires to applying thermal shock or mechanical shock to the substrate in order to cause cracks, however, there is a possibility that the substrate itself may be broken. Thus, this method cannot be generally used for analyzing hexavalent chromium.
In addition, according to the RoHS directive, it is required to determine that particular hazardous substances are not contained in an electronic material. In compliance with the RoHS directive, it is expected to develop an analytical method capable of measuring hazardous elements such as Br, Sb, As, Bi and Pb easily and precisely and a method for controlling the hazardous elements precisely using the analytical method.
In order to non-destructively analyze hazardous elements in an encapsulating resin of a resin encapsulation semiconductor device, use of a fluorescent X-ray analyzer is easy and effective. The fluorescent X-ray analysis generally employs a fundamental parameter (FP) method for a metal sample and a calibration curve method for a resin sample. In order to prepare the calibration curve, standard resin samples are used to which a known quantity of element to be analyzed is added. The standard resin samples having a matrix resin such as polyethylene, ABS resin and vinyl chloride resin are commercially available.
Since the encapsulating resin is compressed in molding, it has a higher density than the standard resin samples. In addition, the encapsulating resin contains filler such as silica for enhancing strength. Thus, a primary X-ray is hard to penetrate into the encapsulating resin, which tends to bring about a lower analytical value of a fluorescent X-ray. Further, when the encapsulating resin of the resin encapsulation semiconductor device is analyzed with a fluorescent X-ray analyzer, there is a possibility to detect a fluorescent X-ray of constituent elements of a lead frame, semiconductor chip and wire, which may be a cause of an error for a fluorescent X-ray of some hazardous element to be detected.
Conventionally, there is proposed a method for judging whether a sample such as polystyrene contains lead (Pb) by fluorescent X-ray analysis. See JP-A 2007-3331 (KOKAI). The method comprises applying an X-ray to a sample, preparing a fluorescent X-ray spectrum, and judging that the sample contains Pb when the spectrum has peaks at all energy positions corresponding to Pb. The method aims at analyzing Pb precisely by avoiding influence of As or Br having a peak overlapping the peak of Pb. However, the method cannot solve the problem of precision due to the lower analytical value for an encapsulating resin of a resin encapsulation semiconductor device which is compressed and has a high density as well as contains filler.
Also, there is known a method for judging a material containing a particular substance in an object to be measured in which materials are formed in layers by fluorescent X-ray analysis. See JP-A 2007-17306 (KOKAI). This publication discloses a sample of Fe on which Zn plating and Cr plating are deposited as the object of layered structure and particular substances such as Cd, Pb, Hg, Br, Cr, Au, Ag, Pt and Pd. The method comprising applying an X-ray to a sample at a controlled depth by varying X-ray irradiation conditions, detecting a fluorescent X-ray emitted from the sample, and judging the material in the layered structure in which the particular substance is contained based on synchronous detention of the material and particular substance. The method is effective for a sample having a relatively simple layered structure and capable of providing sufficient fluorescent X-ray intensity. However, the method cannot solve the problem of precision due to the lower analytical value for an encapsulating resin of a resin encapsulation semiconductor device which is compressed and has a high density as well as contains filler. In addition, the method is not suited for analyzing an encapsulating resin of a resin encapsulation semiconductor device in which a lead frame, chip and wire are arranged into a complicated structure in a matrix resin.